Definitions from Wordnet 2.0

Definitions from Wiktionary

Homicide Meaning and Definition in Wikipedia

Murder is when one person kills another person on purpose. It is only called "murder" when it is against the law. If a person does something that makes someone else die, it is often known as manslaughter or homicide. This is true if it was an accident. Sometimes, a death caused by someone else may not be a crime. For example, in some situations, killing may be self-defense. A person who commits murder is called a murderer.

The legal definition of "murder" and "manslaughter" may be different in different countries, and is very much argued on: for example, killing in war is not usually called "murder" by those who take place in the war. Killing in self defense (if people being attacked kill someone who is attacking them) is usually not "murder".